Coatings for transparent substrates in electronic devices

ABSTRACT

An electronic device may have a housing surrounding an interior in which electrical components are mounted. A display may be mounted to housing structures in the device. The housing may have a rear wall. The display cover layer and rear wall of the housing may be formed from transparent glass layers. Coatings may be formed on inwardly facing surfaces of the transparent glass layers. A coating on a transparent glass layer may be formed from a thin-film interference filter having a stack of dielectric layers. The coating may include an ink layer on the thin-film interference filter.

This application claims the benefit of provisional patent applicationNo. 62/556,243, filed Sep. 8, 2017, which is hereby incorporated byreference herein in its entirety.

FIELD

This relates generally to electronic devices and, more particularly, tocoatings for transparent substrates in electronic devices.

BACKGROUND

Electronic devices often contain displays. A display may have an activearea with pixels that display images for a user and an inactive areaalongside the active area. A layer of glass may serve as a protectivedisplay cover layer. The layer of glass may overlap the active area andthe inactive area. A layer of glass may also form part of a housing foran electronic device. To hide internal components from view, surfaces inan electronic device such as the inner surface of a layer of glassforming a housing for an electronic device and the inner surface of theprotective display cover layer in the inactive area of a display may becovered with a layer of ink.

It may be desirable to improve the outward appearance of the displaycover layer in the inactive area or the output appearance of a glasshousing layer. This can be challenging, because glass is sensitive tostress. If care is not taken, a coating on a glass layer in anelectronic device may make the glass layer susceptible to cracking. Itcan also be difficult to control the appearance of coating layers, whichcan make it difficult to manufacture electronic devices of uniformappearance.

SUMMARY

An electronic device may have a housing in which a display is mounted.The housing may be formed from housing structures that surround aninterior region in the electronic device. Electrical components may bemounted in the electronic device interior.

The display may be coupled to the housing structures on a front face ofthe electronic device. The housing structures may include a rear wall onan opposing rear face of the electronic device.

A display cover layer for the display may have a surface that faces theinterior of the housing. The rear wall may also have a surface thatfaces the interior of the housing. Structures in the electronic devicesuch as the display cover layer and rear housing wall may be formed fromtransparent glass layers. Coatings may be formed on the inwardly facingsurfaces of the transparent glass layers.

A coating on a transparent glass layer may be formed from a thin-filminterference filter having a stack of dielectric layers. The coating mayalso include an ink layer on the thin-film interference filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an illustrative electronic device inaccordance with an embodiment.

FIG. 2 is a cross-sectional side view of a portion of an illustrativeelectronic device having transparent substrates with coatings inaccordance with an embodiment.

FIG. 3 is a cross-sectional side view of a coating layer formed from amultilayer dielectric stack in accordance with an embodiment.

FIG. 4 is a graph of light transmission spectrums for illustrativecoating layers in accordance with an embodiment.

FIG. 5 is a cross-sectional side view of an illustrative transparentsubstrate having an interior surface coated with an illustrative coatingin accordance with an embodiment.

FIG. 6 is a cross-sectional side view of an illustrative transparentsubstrate having an interior surface coated with another illustrativecoating in accordance with an embodiment.

FIG. 7 is a perspective view of an illustrative electronic device havinghousing walls surrounding a coated transparent glass layer in accordancewith an embodiment.

FIG. 8 is a cross-sectional side view of a portion of the electronicdevice of FIG. 7 in accordance with an embodiment.

DETAILED DESCRIPTION

Electronic devices such as cellular telephones often include glassmembers such as display cover glass layers and glass housing members.These layers are traditionally coated with materials such as ink. Theink may be opaque to hide internal device components from view, but maynot always have a desired appearance. The appearance of glass layers inan electronic device can be altered by depositing inorganic layers suchas physical vapor deposition (PVD) inorganic layers onto the glasslayers. Challenges arise, however, in ensuring that the deposited layersproduce desired optical effects (e.g., desired transmission, opacity,and reflection values at various viewing angles) while minimizingundesired manufacturing variations.

To address these challenges, a device such as electronic device 10 ofFIG. 1 may have transparent glass layers or other substrates coated withcoatings that include thin-film interference filters and ink layers. Inthese coatings, thin-film interference filter layers may be arranged toproduce non-neutral colors or to produce neutral colors. The thin-filminterference filter layers may be coated with ink such as neutrallycolored ink or ink with a non-neutral color. Optional buffer layermaterial may be included in the coatings. In some configurations,thin-film interference layers may be supported by a polymer film andattached to a transparent glass layer using a layer of adhesive.

An illustrative electronic device of the type that may have one or morecoated structures is shown in FIG. 1. The coated structures in device 10of FIG. 1 may include transparent structures such as transparent glassstructures (e.g., transparent glass substrates or other transparentsubstrates that form display cover layers, rear housing walls, otherhousing structures, camera windows, lenses with curved surfaces and/orother curved members, and/or other structures). If desired, othersubstrates may be coated (e.g., opaque structures, structures formedfrom materials other than glass, etc.). Illustrative configurations inwhich transparent glass substrates in device 10 are coated are describedherein as an example.

Coated substrates such as transparent glass substrates may be orientedin device 10 so that the coatings face outwardly or inwardly. Forexample, coatings may be located on the inner (interior) surfaces of thesubstrates (the sides of the substrates facing inwardly into theinterior of device 10) so that these coatings may be viewed through thesubstrates from outside the device.

Electronic device 10 may be a computing device such as a laptopcomputer, a computer monitor containing an embedded computer, a tabletcomputer, a cellular telephone, a media player, or other handheld orportable electronic device, a smaller device such as a wristwatchdevice, a pendant device, a headphone or earpiece device, a deviceembedded in eyeglasses or other equipment worn on a user's head, orother wearable or miniature device, a television, a computer displaythat does not contain an embedded computer, a gaming device, anavigation device, an embedded system such as a system in whichelectronic equipment with a display is mounted in a kiosk or automobile,equipment that implements the functionality of two or more of thesedevices, an accessory (e.g., earbuds, a remote control, a wirelesstrackpad, etc.), or other electronic equipment. In the illustrativeconfiguration of FIG. 1, device 10 is a portable device such as acellular telephone, media player, tablet computer, or other portablecomputing device. Other configurations may be used for device 10 ifdesired. The example of FIG. 1 is merely illustrative.

In the example of FIG. 1, device 10 includes display 14. Display 14 hasbeen mounted in housing 12. Housing 12, which may sometimes be referredto as an enclosure or case, may be formed of plastic, glass, ceramics,fiber composites, metal (e.g., stainless steel, aluminum, etc.), othersuitable materials, or a combination of any two or more of thesematerials. Housing 12 may be formed using a unibody configuration inwhich some or all of housing 12 is machined or molded as a singlestructure or may be formed using multiple structures (e.g., an internalframe structure, one or more structures that form exterior housingsurfaces, bezel structures, housing sidewall structures, rear housingwalls formed from glass plates or other planar transparent members,metal, plastic, and/or other materials, and/or other housing members).Openings may be formed in housing 12 to form communications ports, holesfor buttons, and other structures.

Display 14 may be a touch screen display that incorporates a layer ofconductive capacitive touch sensor electrodes or other touch sensorcomponents (e.g., resistive touch sensor components, acoustic touchsensor components, force-based touch sensor components, light-basedtouch sensor components, etc.) or may be a display that is nottouch-sensitive. Capacitive touch sensor electrodes may be formed froman array of indium tin oxide pads or other transparent conductivestructures.

Display 14 may have a central active area that includes an array ofpixels. The array of pixels may be formed from liquid crystal display(LCD) components, an array of electrophoretic pixels, an array of plasmadisplay pixels, an array of organic light-emitting diode pixels or otherlight-emitting diode pixels, an array of electrowetting pixels, orpixels based on other display technologies. In some configurations, aninactive border area that is free of pixels may run along one or moreedges of display 14.

Display 14 may be protected using a display cover layer such as a layerof transparent glass, clear plastic, transparent ceramic, sapphire orother transparent crystalline material, or other transparent layer(s).The display cover layer may have a planar shape, a convex curvedprofile, a concave curved profile, a shape with planar and curvedportions, a layout that includes a planar main area surrounded on one ormore edges with a portion that is bent out of the plane of the planarmain area, or other suitable shape. If desired, one or more openings maybe formed in the display cover layer to accommodate optional componentssuch as button 16, ports such as speaker port 18, and other structures.In some configurations, display 14 may have an outer layer such as acolor filter layer or a thin-film transistor layer in a liquid crystaldisplay that is sufficiently thick and strong to serve as a displaycover layer. In other configurations, the outermost layer of display 14may be a separate cover layer that does not have any color filterelements or thin-film transistor circuitry.

Illustrative device 10 of FIG. 1 has a rectangular footprint (outlinewhen viewed from above) with four peripheral edges. Housing 12 may havesidewalls 12SW that run along the four peripheral edges of device 10.Sidewalls SW may be vertical sidewalls, curved sidewalls, integralportions of a rear housing wall that extend fully or partly up the sidesof housing 12, and/or other suitable sidewall structures. In someconfigurations, display 14 has peripheral portions that extend down someor all of the side edges of device 10. Housing 12 may have a rear wallsuch as rear wall 12R. Rear wall 12R may be formed from integralportions of sidewalls 12SW and/or from separate structures. Rear wall12R may have a substantially planar surface on a rear face of device 10.Display 14 may include a parallel planar surface on the opposing frontface of device 10.

A cross-sectional side view of device 10 taken along line 20 and viewedin direction 22 of FIG. 1 is shown in FIG. 2. As shown in FIG. 2, device10 may have an interior in which electrical components 50 are housed.Electrical components 50 may include integrated circuits, sensors, andother circuitry. Components 50 may be mounted on one or more printedcircuits such as printed circuit 48.

Display 14 of FIG. 2 may have a transparent layer such as display coverlayer 30 (i.e., the outermost layer of display 14). Display cover layer30 may be formed from a transparent material such as glass, plastic,sapphire or other crystalline material, transparent ceramic, etc. In theactive area of display 14, display 14 may contain pixel array structures32 (e.g., an organic light-emitting diode display layer, a liquidcrystal display module, etc.) with an array of pixels 31 for displayingimages.

Rear housing wall 12R may be formed from a planar member such as atransparent glass substrate (transparent glass member 34). Transparentglass substrates such as display cover layer 30 and/or member 34 may beprovided with coatings. In the example of FIG. 2, the underside ofdisplay cover layer 30 in the inactive area of display 14 has beencoated with coating 28. The inner surface of member 34 (e.g., a glassplate) has been provided with coating layer 36 and coating 52. Coating52 may be formed from the same coating materials as coating layer 36and/or may be formed differently so that coating 52 has a differentvisual appearance than coating 36. Coating 52 may, as an example, bepatterned to form text, a logo, or other suitable visual element on therear of housing 12. A user such as viewer 42 who is viewing the frontface of device 10 in direction 40 may view coating 28 through displaycover layer 30. A user such as viewer 46 who is viewing the rear face ofdevice 10 in direction 44 may view coatings such as coating 36 andcoating 52 through member 34. If desired, sidewalls 12SW may be formedfrom transparent glass structures (e.g., sidewall members or portions oflayer 30) and coatings such a coatings 28, 36, and/or 52 may be formedon the inner surfaces of these members (as an example).

Coatings such as coatings 28, 26, and 52 may be formed from dielectriclayers, metal layers, and/or other layers of material. These layers maybe deposited by spraying, printing (e.g., screen printing, inkjetprinting, pad printing, etc.), dripping, painting, chemical vapordeposition (e.g., plasma enhanced chemical vapor deposition,), physicalvapor deposition (e.g., evaporation and/or sputtering), atomic layerdeposition, electroplating, lamination, and other deposition techniques.Coatings may be patterned using shadow mask deposition, printingpatterning techniques, photolithography (lift-off, etching, etc.), laserpatterning (e.g., ablation), mechanical patterning (e.g., drilling,grinding, milling, etc.) and/or other patterning techniques.

In some arrangements, multiple thin-film layers for a coating may beformed in a stack. Thin-film stacks such as these may form thin-filminterference filters (sometimes referred to as dichroic filters ordichroic layers). The optical properties of each of the layers in athin-film stack (e.g., the index of refraction of each layer) and thethickness of each layer may be selected to provide the thin-filminterference filter with desired characteristics (e.g., a desired lighttransmission spectrum, a desired light reflection spectrum, a desiredlight absorption spectrum). These characteristics may provide a coatingwith a desired appearance when present on the inner surface of atransparent substrate (e.g., a desired color, etc.). A thin-film stackmay, as an example, be configured to reflect light of a particular coloror to exhibit a color-neutral behavior (e.g., to serve as aneutral-color partially reflective mirror).

FIG. 3 is a cross-sectional side view of an illustrative thin-film stackconfigured to form a thin-film interference filter. The thin-film stackof FIG. 3 has multiple layers 56. Layers 56 may have thicknesses of0.01-1 micron, at least 0.05 microns, at least 0.1 microns, at least0.15 microns, less than 1.5 microns, less than 1 micron, etc. Layers 56may be inorganic dielectric layers (e.g. oxides such as silicon oxide,niobium oxide, titanium oxide, tantalum oxide, zirconium oxide,magnesium oxide, etc., nitrides such as silicon nitride, oxynitrides,and/or other inorganic dielectric materials). Organic dielectric layers(e.g., clear polymer layers) and/or other materials (thin metal films,semiconductor layers, etc.) may also be included in the thin-film stack,if desired.

In the example of FIG. 3, the thin-film stack formed from layers 56forms thin-film interference filter 54. Filter 54 may be formed formdielectric materials such as inorganic dielectric layers deposited withphysical vapor deposition techniques and may therefore sometimes bereferred to as a physical vapor deposition layer, physical vapordeposition coating, or physical vapor deposition stack. Other techniquesfor forming filter 54 may be used, if desired.

Filter 54 may be configured to exhibit high reflectivity (e.g., filter54 may be configured to form a dielectric mirror that reflects arelatively large amount of light (see, e.g., reflective light I2)relative to incident light I1, may be configured to exhibit lowreflectivity (e.g., filter 54 may be configured to form anantireflection coating so that a relatively large amount of light I3passes through filter 54 relative to incident light I1), may beconfigured to form a colored (tinted) layer (e.g., by reflecting one ormore selected colors of light such as when configuring filter 54 toserve as a bandpass filter, band-stop filter, low pass filter, or highpass filter), and/or may be configured to from a light-blocking layer(e.g., by exhibiting a high opacity). Layers 56 may also be configuredto adjust the optical properties (transmission, reflection, absorption)of filter 54 at multiple different values of angle A (e.g., an angle Awith respect to surface normal n for filter 54 that is associated withan incident angle of incoming light such as ray R1 and that is alsoassociated with corresponding angle of view for a viewer viewingreflected light such as ray R2).

FIG. 4 is a graph containing curves 60 and 58 for two respectiveillustrative light transmission spectrums for filter 54 (e.g., visiblelight transmission spectrums at an illustrative angle A of 0°). As shownby illustrative curve 58, there may be complex features at multipledifferent wavelengths (e.g., peaks, valleys, etc.) in the lighttransmission spectrum for filter 54 (e.g., over visible lightwavelengths k). In other configurations (e.g., curve 60), filter 54 isconfigured to exhibit a neutral color spectrum.

The optical characteristics of filter 54 can be tuned (at one or morevalues of angle A) by adjusting the attributes of layers 56 (e.g., indexof refraction, thickness, etc.). The optical properties of filter 54 mayalso be adjusted by adjusting the number of layers 56 in filter 54. Withone illustrative configuration, the overall thickness of filter 54 ismaintained at a relatively low value (e.g., 80-300 nm, less than 3microns, less than 2 microns, less than 1 micron, at least 0.1 micron)by limiting the thicknesses of each of layers 56 (e.g., to less than 1.5microns, less than 1 micron, less than 0.5 microns, less than 0.4microns, etc.) and by limiting the number of layers 56 in filter 54(e.g., to 2-6, at least 2, at least 3, at least 4, at least 5, fewerthan 20, fewer than 14, fewer than 10, fewer than 7, etc.). In general,filter 54 need not be restricted to these configurations and may containany suitable types of layers 56 and/or may include layers 56 of anysuitable thickness, index of refraction, etc.

In some arrangements, it may be desirable for filter 54 to be configuredto exhibit a color tint (in reflection and/or transmission). Forexample, it may be desirable for filter 54 to reflect red light so thatfilter 54 has a pink color or to reflect light that provides filter 54with a gold appearance in reflection. In other arrangements, it may bedesirable for layer 54 to exhibit a neutral color (e.g., white, gray,black, etc.) and/or a color that is relatively constant in color castover a wide range of angles A (e.g., a wide range of viewing angles).

The apparent color of filter 54 may be characterized by a color in Labcolor space. With one illustrative configuration, filter 54 operates asa partially reflective mirror (e.g., a mirror of 10-20% reflectivity, ora reflectivity of at least 5%, at least 15%, at least 20%, less than85%, less than 60%, less than 50%, less than 35%, or other suitablevalue) and exhibits a gray color in reflection (e.g., the reflectivityof filter 54 is neutral in color so that filter 54 forms a color-neutralpartially reflective mirror). In this configuration, for example, thecolor of reflected light may be characterized by Lab color coordinates aand b that are less in magnitude than 5, less in magnitude than 3, orother suitable neutral values (e.g., the value of color coordinate “a”may be about −1 and the value of color coordinate “b” may be about −2).If desired, filter 54 may be configured to exhibit an angularlyinvariant color. For example, the changes in the magnitudes of colorcoordinates a and b (e.g., Δa and Δb, respectively) may be maintained atvalues less than 2, less than 3, less than 4, or other suitable valuesover a range of viewing angles (reflected light angle A) of 0-60°.

Layers 56 may include inorganic materials such as oxides. For example,layers 56 may include one or more silicon oxide layers and one or moreniobium oxide layers. Niobium oxide can be deposited consistently usingsputtering and may allow filter 54 to exhibit good color control. Otheroxides may be used (e.g., one or more tantalum oxide layers 56 may beinterspersed with one or more silicon oxide layers in filter 54, one ormore titanium oxide layers 56 may be interspersed with one or moresilicon oxide layers, etc.). In some arrangements, higher and lowerrefractive index materials alternate in the stack of layers formingfilter 54. For example, filter 54 may include alternating niobium oxidelayers and silicon oxide layers, may include alternating titanium oxideand silicon oxide layers, or may include alternating tantalum oxidelayers and silicon oxide layers.

Filter 54 may form part of a coating on a transparent glass substrate indevice 10. In this type of configuration, the most inwardly facing layer56 of filter 54 (e.g., the last layer 56 that is deposited on filter 54in an illustrative configuration in which filter 54 is formed on atransparent glass substrate) may be formed from a layer of silicon oxideto enhance adhesion with subsequent layers such as a subsequent inklayer. The ink layer may be a polymer containing colorant such as dyeand/or pigment. The colorant may have a neutral color such as white,gray, or black, may have a non-neutral color such as red, blue, green,yellow, gold, or may have another suitable color.

A cross-sectional side view of an illustrative coated substrate fordevice 10 is shown in FIG. 5. As shown in FIG. 5, substrate 68 (e.g., atransparent glass substrate, etc.) may have an outer surface such asouter surface (exterior surface) 90 that faces a user such as viewer 62who is viewing substrate 68 in direction 64 and that therefore faces theexterior regions surrounding device 10. Substrate 68 may also have anopposing inner surface 90 (interior surface) that faces the interior ofdevice 10 and housing 12 away from viewer 62. Coating 74 may be formedon inner surface 92 and may face the interior of housing 12 and device10. Substrate 68 may be, for example, substrate 30 of FIG. 2, substrate34 of FIG. 2, and/or other substrate in device 10. Coating 74 may becoating 28 of FIG. 2, coating 36 of FIG. 2, coating 52 of FIG. 2, and/oranother coating.

In the illustrative configuration of FIG. 5, thin-film interferencefilter 54 has initially been deposited on a separate substrate(substrate 76). Substrate 76 may be, for example, a sheet of polymer.After forming reflective layer 72 by forming thin-film interferencefilter 54 on flexible substrate 76, reflective layer 72 may be laminatedto inner surface 92 of substrate 68 using a layer of adhesive such asadhesive layer 70 (e.g., a polymer layer). Optional ink layer 78 may beformed on the interior surface of layer 72 (e.g., on the inner surfaceof substrate 76). If desired substrate 76 may be omitted to help reducethe thickness of layer 74 (e.g., the layers of filter 54 may form layer72 without using an additional polymer film substrate such as substrate76). Filter 54 of FIG. 5 may be formed using 2-6 layers 56 or any othersuitable number of layers 56. Filter layers in device 10 such as filter54 may be patterned to form logos, text, icons, and/or other patterns.

Another illustrative arrangement for providing substrate 68 with acoating on interior surface 92 is shown in FIG. 6. In the example ofFIG. 6, coating 80 has been formed on inner surface 92 of substrate 68.Viewer 62 may view coating 80 in direction 64 through the transparentmaterial of substrate 68.

Substrate 68 may be coated with an optional buffer layer such as layer70. Layer 70 may be a clear polymer and may have a thickness of 50 nm-3microns, at least 0.1 microns, at least 0.2 microns, at least 0.3microns, at least 0.5 microns, at least 1 micron, at least 2 microns,1-3 microns, less than 5 microns, or other suitable thickness. Whenfilter 54 is relatively thick, the presence of optional buffer layer 70may help reduce stress-induced cracks that might damage substrate 68. Ifthe amount of stress imparted by filter 54 is relatively low, bufferlayer 70 may be omitted.

Ink layer 78 may be deposited on the inner surface of filter 54. Filter54 may include layers 56 such as inorganic dielectric layers ofalternating refractive index values. The layer 56 in filter 54 that isimmediately adjacent to ink layer 78 may be formed from silicon oxide topromote adhesion (e.g., to ensure that ink layer 78 securely adheres tolayer 54). When ink layer 78 is formed from white material or otherbrightly colored material, ink layer 78 may help reflect light that hasbeen transmitted through substrate 68 outwardly towards viewer 62. Whenink layer 78 is black, light transmitted through substrate 68 may beabsorbed, so that the color of the light related from filter 54 towardsviewer 62 dominates. Gray ink reflects some but not all of the lightthat has been transmitted through substrate 68. In configurations inwhich ink 78 has a non-neutral color (e.g., red, green, blue, yellow,gold, etc.), the color of coating 80 will be tinted accordingly. Byusing a color-neutral and angularly insensitive design for filter 54(e.g., a gray mirror with a reflectively of 10-30%), filter 54 andtherefore coating 80 will be relatively insensitive to performancefluctuations due to manufacturing variations in layers 56. This helpsensure consistency when manufacturing numerous devices 10.

If desired, the overall thickness of layer 80 may be minimized by usinga relatively small number of layers 56 in filter 54. This approach maybe facilitated by using ink 78 to provide coating 80 with a desiredcolor rather than relying on filter 54 to impart the desired color.

Other configurations may be used for coating 80, if desired. Theconfiguration of coating 80 described in connection with FIG. 6 ismerely illustrative. Buffer layer 70 may be included in coating 80 ormay be omitted. Filter 54 may form a neutral-color angularly insensitivethin-film interference filter or may have a color and/or exhibit angularchanges in color. Filter 54 may be configured to serve as a highlyreflective mirror, a partially reflective mirror, an antireflectioncoating, etc. Layers 56 of filter 54 may be deposited by physical vapordeposition and/or other techniques.

The properties of the substrate coatings in device 10 may allow thesubstrate to be visually matched to nearby structures such as portionsof housing 12 at one or more viewing angles. Consider, as an example,the arrangement of device 10 of FIG. 7. As shown in FIG. 7, substrate100 (e.g., substrate 30, substrate 34, or other suitable substrate indevice 10) may be mounted adjacent to a band of housing structures orother exposed portions of housing 12. FIG. 8 is a cross-sectional sideview of device 10 of FIG. 7 showing how substrate 100 may have a coatingsuch as coating 106 (e.g., coating 74 of FIG. 5 or coating 80 of FIG.6).

Coating 106 may include thin-film interference filter 54 and optionalink 78 that are selected to provide substrate 100 with an appearancethat matches that of housing 12 and/or that contrasts or otherwisecomplements that of housing 12. This matching (or contrasting) may occurat one or more viewing angles A. For example, the appearance of housing12 and coated substrate 100 may match (or contrast in a predeterminedfashion) when a viewer such as viewer 102 is viewing substrate 100 atnormal incidence and may contrast in a predetermined fashion (or match)when viewer 102 is viewing substrate 100 in direction 104 at an off-axisangle A (e.g., when A is not zero and has another value such as a valueof at least 45°).

As an example, housing 12 may have a gold color and coating 106 ofsubstrate 100 may have a configuration that provides substrate 100 witha gold appearance (e.g., at an on-axis viewing angle where A is zero orat an off-axis viewing angle such as when A is at least 45°). In thistype of configuration, the appearances of substrate 100 and housing 12are deliberately blended.

As another example, consider a scenario in which the appearance ofcoated substrate 100 is configured to be gold at an on-axis viewingangle and in which ink 78 has a contrasting color such as blue. Whensubstrate 100 is viewed at normal incidence (on-axis), a relativelysmall amount of underlying blue color from ink 78 will be visible sosubstrate 100 will appear to be gold. When substrate 100 is viewed at anoff-axis viewing angle (e.g., at least 45°), however, the blue color ofthe underlying ink 78 in coating 106 will be visible to the user throughcoating 106. As a result, the appearance of substrate 100 (e.g., rearhousing wall 12R, etc.) will change from gold to blue, depending on theangle at which substrate 100 is viewed.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. An electronic device having an interior and having opposing front and rear faces, comprising: housing structures; a display coupled to the housing structures; a transparent layer that overlaps the display and forms the front face; a transparent glass substrate coupled to the housing structures, wherein the transparent glass substrate is a glass housing member on the rear face; and a coating across the transparent glass substrate that faces the interior and that is viewable through the transparent glass substrate, wherein the coating comprises a layer of ink and a thin-film interference filter interposed between the layer of ink and the transparent glass substrate, wherein the thin-film interference filter forms a partially reflective mirror, and wherein the partially reflective mirror has a reflectivity that is neutral in color.
 2. The electronic device defined in claim 1 wherein the thin-film interference filter is configured to exhibit color coordinate values a and b in Lab color space that are each less than 5 in magnitude.
 3. The electronic device defined in claim 2 wherein the ink layer has a non-neutral color.
 4. The electronic device defined in claim 1 wherein the thin-film interference filter is configured to exhibit color coordinate value changes Δa and Δb in Lab color space that are each less than 2 in magnitude over a range of incident light angles relative to the thin-film interference filter of 0° to 60°.
 5. The electronic device defined in claim 4 wherein the thin-film interference filter comprises 2-6 inorganic dielectric layers.
 6. The electronic device defined in claim 5 wherein the thin-film interference filter includes at least one silicon dioxide layer.
 7. The electronic device defined in claim 6 wherein the ink layer adheres to the silicon dioxide layer.
 8. The electronic device defined in claim 1 wherein the thin-film interference filter comprises a niobium oxide layer.
 9. The electronic device defined in claim 1 wherein the thin-film interference filter comprises a polymer substrate and a stack of multiple dielectric layers of alternating refractive index on the polymer substrate.
 10. The electronic device defined in claim 1 wherein the thin-film interference filter comprises a stack of multiple dielectric layers attached to the transparent glass substrate with a layer of adhesive.
 11. The electronic device defined in claim 1 further comprising a clear polymer buffer layer between the thin-film interference filter and the transparent glass substrate.
 12. An electronic device having an interior and having opposing front and rear faces, comprising: housing structures; a transparent substrate coupled to the housing structures on the rear face; and a coating across the transparent substrate that faces the interior and that is viewable through the transparent substrate, wherein the coating comprises a layer of ink and a stack of multiple inorganic dielectric layers between the layer of ink and the transparent substrate, wherein the stack of multiple inorganic dielectric layers exhibits color coordinate value changes Δa and Δb in Lab color space over ranges of incident light angles relative to the stack of multiple inorganic dielectric layers, and wherein a maximum color coordinate value change in a and b coordinates in Lab color space across all incident light angles between 0° to 60° is less than 2 in magnitude.
 13. The electronic device defined in claim 12 wherein the multiple inorganic dielectric layers include at least one silicon oxide layer and wherein the layer of ink adheres to the silicon oxide layer.
 14. The electronic device defined in claim 13 wherein there are fewer than 7 of the inorganic dielectric layers between the layer of ink and the transparent substrate.
 15. The electronic device defined in claim 14 wherein the transparent substrate comprises a layer selected from the group consisting of: a display cover glass layer and an electronic device housing wall layer.
 16. The electronic device defined in claim 15 wherein the inorganic dielectric layers include at least one niobium oxide layer.
 17. Apparatus comprising: a glass layer having a surface configured to face an interior of a rear face of an electronic device housing; and a coating layer across the glass layer, wherein the coating layer includes a thin-film interference filter formed from a stack of dielectric layers, a layer of adhesive that attaches the stack of dielectric layers to the surface of the glass layer, and an ink layer on the stack of dielectric layers, wherein the stack of dielectric layers exhibits color coordinate value changes Δa and Δb in Lab color space over ranges of incident light angles relative to the stack of dielectric layers, wherein a maximum color coordinate value change between incident light angles of 0° to 60° is less than 5 in magnitude, and wherein the stack of dielectric layers exhibits a neutral color in reflection for all incident light angles between 0° and 60°.
 18. The apparatus defined in claim 17 further comprising a polymer film having first and second opposing surfaces, wherein the stack of dielectric layers is formed on the first surface and wherein the ink layer is formed on the second surface.
 19. The apparatus defined in claim 17 wherein the glass layer is selected from the group consisting of: a display cover layer and an electronic device housing wall. 